Method for determination of impurities in helium gas

ABSTRACT

The helium content of a gaseous mixture is determined by comparing the partial pressure of the helium in an unknown sample directly with the pressure of highly purity helium under the same conditions. The volume percent of helium in the unknown is equal to(P1/P2) X 100, where P1 is the partial pressure of helium in the sample and P2 is the total pressure of an equal volume of high-purity helium. The partial pressure of the helium in the unknown sample is initially determined by adsorption of gases other than helium on activated charcoal and measurement of the resulting helium pressure. The total pressure of high-purity helium is then measured under the same conditions as the sample.

United States Patent Emerson [451 Sept. 19, 1972 [54] METHOD FORDETERMINATION OF IMPURITIES IN HELIUM GAS [72] Inventor: Ilavid E. Emersg, Amarillo Tex,

[73] Assignee: The United States of America, as represented by theSecretary of the Interior [22] Filed: March 27, 1970 [21] Appl. No.:23,133

[52] U.S.Cl ..73/23 [51] Int. Cl. ..G01n 7/04 [58] Field of Search..73/23, 23.1, 29; 23/232 C, 23/254 R, 254 E, 255 R, 255 E [56]References Cited UNITED STATES PATENTS I 3,311,454 3/1967 Kemeny ..73/23X 3,557,604 1/1971 Baecklund ..73/19 2,601,272 6/1952 Frost ..73/233,135,108 6/1964 Santeler ..73/23.l 3,211,006 10/1965 I-Ialey ..73/23.1

OTHER PUBLICATIONS A. L. M. Keulemans, Gas Chromatography (ReinholdPublishing Corporation, New York, 1957) pp. 70, 71, 95.

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C. L. Klingman, Modified Chromatograph to Record Helium Content ofNatural Gas Streams, The Review of Scientific Instruments, Vol. 32, No.7, 7/61; 822- 4.

Primary Examiner-Richard C. Queisser Assistant Examiner-C. E. Snee, IIIAttorney-Ernest S. Cohen and William S. Brown [5 7] ABSTRACT The partialpressure of the helium in the unknown sample is initially determined byadsorption of gases other than helium on activated charcoal andmeasurement of the resulting helium pressure. The total pressure ofhigh-purity helium is then measured under the 1 Claim, 1 Drawing FigureVENT V VACUUM PATENTEU I97? 3.691.818

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V VACUUM //VVE/VTOR DAV/0 E. EMERSON M6921 M M- ATTORNEYS METHOD FORDETERMINATION OF IMPURITIE IN HELIUM GAS Various methods have beenconventionally employed for analysis of the helium content of crudehelium, particularly mass spectrometry and the procedure employing theModified Frost Apparatus, described in Bureau of Mines Report ofInvestigations 6545. Neither of these, however, are primary standardmethods, and both are expensive and require extensive calibration.

It has now been found, according to the method of the invention, thatthe helium content of a gaseous mixture may be accurately andeconomically determined by a procedure comprising (1) determining thepartial pressure of the helium in the unknown sample and (2) determiningthe total pressure of high-purity helium under the same conditions. Thevolume percent of helium in the unknown is then calculated from theequation: %He,.,,, -P,/P 'X 100, where P is the partial pressure ofhelium in the unknown and P is the total pressure of an equal volume ofhigh-purity helium. This results in a primary standard method ofdetermining the helium content of samples containing. from about 0.1 to100 percent helium with an accuracy of :0.07 percent. The method of theinvention may be used for analysis of helium-containing natural gases,crude helium from industrial sources, helium in gases used in research,etc.

The partial pressure of the helium in the unknown is determined byadsorption of the gaseous components, other than helium and neon, onactivated coconut charcoal at liquid nitrogen temperature (77 K). Sincethe neon is generally present only in the parts-per-million range, itspressure may be neglected. This method for adsorption of gases otherthan helium is conventional and is disclosed in the above-mentionedReport of Investigations 6545.

The helium is treated as an ideal gas because the total pressure in theapparatus employed does not exceed one atmosphere. According to DaltonsLaw, the total pressure of a gaseous mixture is equal to the sum of thepartial pressures of the components. The partial pressure is afictitious pressure which each pure component would exert if it aloneoccupied the entire volume. The method of the invention determines thepartial pressure of helium in the sample by adsorbing the othercomponents on activated coconut charcoal at 77 K.

Therefore, if

P, the partial pressure of helium in the sample and P the total pressure(a volume of high-purity helium equal to the volume of the unknownsample), then,

P,/P X 100 percent helium in sample.

I A total helium pressure measurement is required after each adsorptioncycle. The adsorption of the gases other than helium changes theinternal volume of the system, and the total pressure must be determinedat the same volume as the partial pressure of helium in the sample.

In addition, it has been found that optimum precision is obtained by theuse of helium-free nitrogen gas to sweep the sample gas into the area ofthe apparatus in which the charcoal is located, and in which thepressure measurements are made. This procedure is more fully explainedin the following detailed description of the invention.

The invention will now be described in detail with reference to theFIGURE, which is a flow diagram of the method of the invention. Detailsof a suitable apparatus for practice of the invention are included inthe following description; however,'the specific apparatus is not theessence of the inventionand many modifications of details of theapparatus will be apparent to those skilled in the art without departingfrom the inventive concept as defined inthe claims.

In addition, maximum speed and efficiency of operation is achieved byproper timing of thevarious stages of the analytical procedure, as morefully described below. However, the exact time intervals employed mayvary considerably, depending on the specific details of the apparatusemployed, and are; readily determined by one skilled in the art.

Operation of the apparatus in the analytical procedure of the inventionis desirably automated by means of an electrical control unit. Flow ofgases may, e.g., be controlled by cams which control solenoidoperatedair-control valves, which in turn control the gas-flow valves. Suchautomation is also conventional in analytical instrumentation and isalso obvious to one skilled in the art. I

In this embodiment of the invention an essential aspect of theinvention, i.e., measurement of the partial pressure of the helium inthe sample and the pressure of the high-purity helium under the sameconditions, is achieved by means of the following stages of operation:(1) collecting the sample gasin a sample loop, (2) pushing the samplegas from the sample loop into a zone where adsorption takes place andpressure measurements are made, (3) collecting the high-purity helium inthe sample loop, and (4) pushing the high-purity helium from the sampleloop into the adsorption and pressure-measuring zone.

STAGE 1 In this stage the sample gas from container 1 flows throughmetering valve 2, line 3 (M; inch steel tubing), valve 4, line 5, valve28, line 6 and sample inject valve 7 to sample loop 8 /4 inch steeltubing, 7 cm volume). From the sample loop thegasflows via line 9 toflowmeter 10 and is then vented to the atmosphere. The metering valve isa 20-turn valve for control of the gas flow to the flowmeter. Valve 4 isa three-port pneumatic on-off valve which is in the on position in thisstage. Valve 7, the sample inject valve, is an eight-port pneumaticvalve which, in this stage, is set to connect lines 6 and 9 with thesample loop. In this stage of the procedure the sample loop is initiallypurged of all gases except the sample and simultaneously filled with thesample gas.

Simultaneously, nitrogen gas flows from container 11 through meteringvalve 12, line 13, three-port on-off valve 14 and line 15 to nitrogenflowmeter l6 and is then vented to the atmosphere. Valve 14 is, ofcourse, set to connect lines 13 and 15. The flow rate of the nitrogen isthus determined at the same time that the sample loop is being filledwith the sample gas.

Also simultaneously, the adsorbent section of the apparatus, i.e., thesection between valve 14 and valve 17 is evacuated to a pressure ofabout 0.1 Torr or less. This section includes line 18, between valves 7and 14, line 19, charcoal trap 20, containing charcoal (3 grams, 50-60mesh activated coconut charcoal) 21, line 22 STAGE 2 After 39 seconds ofoperation stage 2 is begun by closing valve 28. This stops the flow ofsample gas and reduces the sample pressure in the sample loop toatmospheric. In 9 more seconds'valve l7 closes and valve 7 actuates,thereby connecting the lower leg of the sample loop8 with line 19. Inaddition, actuation of valve 7 disconnects line 6 from the sample loop,thereby stopping the flow of sample gas at valve 7. Since the volumebetween valves 14 and 17, i.e., the adsorbent section, has beenevacuated, the initial reaction of the sample, from atmospheric pressureto a vacuum, is to expand into the adsorbent section.

At the same time valve 14 is actuated to connect lines 13 and 18 and theupper leg of the sample loop, thereby diverting the nitrogen flowthrough the sample loop. Flow rate and time of nitrogen flow arecorrelated to provide sufficient nitrogen to push the sample out of theloop and into the adsorbent section. In so doing, the sample passesthrough the charcoal trap where gases other thanhelium and neon areadsorbed. Nitrogen flow rates of l em /minute to 40 cm minute have beenfound to be satisfactory; however, a rate of about 3 cmlminute ispreferred.

:ln 6 more seconds valves 7 and 14 deactivate, trapping the sample intheadsorbent section.

STAGE 3 A l In this stage valves 27 and 28 open, while valve 4 tionafter the charcoal has adsorbed gases other than helium and neon. Thismeasurement is obtained by means of transducer 24', conveniently a 0-1psid transducer with an electrical span set to give a reading of from 0to 1.5 volts for the pressure of helium in the sample.

During this'stage the nitrogen flow is again diverted through valve '14to flowmeter 16 to reconfirm the flowrate of the nitrogen.

After the sample pressure reading is obtained, valve 17 opens and theadsorbent section is again evacuated for 361 seconds. A new cycle isthen started in which the pressure of the pure helium is measured underthe same conditions as that employed in obtaining the pressure of thehelium in thggfag$e4 In this stage valve 17 closes, andafter 5 secondsthe liquid nitrogen is added to the charcoal trap. After another 15seconds valve 28 closes to stop the flow of high-purity helium andreduce the helium pressure to atmospheric. I

After another 9 seconds valves 7 and 14 actuate, as in Stage 2, and thehigh-purity helium from the sample loop is pushed into the adsorbentsection by means of nitrogen. The exact conditions are thus duplicatedfor injecting the sample and the high-purity helium,

respectively, into the adsorbent section.

Valve 7 then deactivates to isolate the high-purity helium from thesample loop in the adsorption section. At the same time a new sample isobtained in the sample loop in the same manner as in Stage 1.

A pressure measurement is then made on the highpurity helium in theadsorption section in the same manner as the measurement of the heliumin the sample in Stage 3.

What is claimed is:

l. A method for analyzing a helium-containing gas comprising (1)isolating a specific volume of sample gas in a sample loop atatmospheric pressure, (2) utilizing a flow of a measured volume ofnitrogen gas to sweep the sample from the sample loop into an enclosedadsorbent section containing activated charcoal, (3) contacting thethus-isolated sample with said charcoal at liquid nitrogen temperaturefor a time sufficient to adsorb gases other than helium, (4) measuringthe pressure of the resulting gas consisting essentially of helium todetermine the-partial pressure of helium in the sample, (5) isolatingthe same volume of high-puri' ty helium in the sample loop atatmospheric pressure, (6) utilizing a flow of the same volume ofnitrogen gas to sweep the high-purity helium from the sample loop intothe same adsorbent section containing the activated charcoal withadsorbed impurities, and (7) measuring the pressure of the thus isolatedhigh-purity helium, whereby the helium content of the sample gas may becalculated from the equation:

% He P,/P, x

where the He, is the percent of helium by volume in the sample, P is thepartial pressure of heliumin the sample, and P, is the pressure of thehigh-purity helium.

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